The field of the invention is fire retardant, fine particulate, expandable styrene polymers for the preparation of molded articles. The present invention is particularly concerned with the production of expandable, particulate molding compositions of styrene polymers.
The state of the art of expandable polystyrene may be ascertained by reference to the Kirk-Othmer, "Encyclopedia of Chemical Technology," 2nd Edition, Vol. 9 (1966) under the section entitled "Foamed Plastics," pages 847-884, particularly pages 852, 853 and 855 where polystyrene is disclosed, and Vol. 19 (1969) under the section entitled "Styrene Plastics," pages 85-134, particularly pages 116-120, where polystyrene foams are disclosed and pages 120, 121 where prior art self-extenguishing polystyrene foams are disclosed and British Pat. Nos. 877,864 and 1,024,195 and U.S. Pat. Nos. 3,060,138; 3,265,643; 3,320,188; 3,389,097; 3,657,162; 3,682,844; 3,696,060; 3,755,209; 4,029,614; 4,086,191; 4,192,922; 4,228,244 and 4,281,036, the disclosures of which are incorporated herein.
The preparation of fine particulate, expandable styrene polymers takes place by a suspension polymerization process. Styrene and possibly styrene plus comonomers containing in dissolved form one or more water insoluble initiators are stirred and dispersed into approximately the same quantity of water. The amount and the kind of polymerization initiators are selected with respect to the temperature of polymerization so that on the one hand a complete polymerizing conversion is achieved in the shortest possible time, and on the other hand a polymer with a molecular weight between 200,000 and 300,000 is obtained. Slight amounts of an organic or inorganic dispersing agent, for instance a water-insoluble protective colloid such as polyvinylalcohol, hydroxyethyl cellulose, or a water-insoluble salt, such as tricalcium phosphate are used to improve the dispersion of the styrene phase in water or to adjust the particle size. In the case of particle size adjustment, slight amounts of an emulsifier are used to stabilize the suspension. Depending on the process, the expanding agent is added before, during or after the polymerization. In particular, the main amount of the expanding agent is added after formation of the grain spectrum, namely after a conversion of 50, in particular at least 70%, as disclosed in British Pat. Nos. 877,864 and 1,024,195; U.S. Pat. Nos. 3,696,060 and 3,657,162 and West German Published Application No. 19 16 387.
After completion of the polymerization cycle, the polymer is separated from the aqueous phase, washed and dried and sifted. Prior to further processing, the crude beads typically are provided with a surface coating for the purpose of preventing bonding during prefoaming.
Molded articles are made by expanding the fine, particulate, expandable styrene polymers in molds. In this procedure the fine, particulate styrene polymers first are heated by steam or hot gases to temperatures above their softening point, whereby they foam into a loose heap and this step is called prefoaming. The prefoamed styrene polymers first are stored for some interim time (ageing step) and then are made to foam further within a pressure-resistant but not gas-tight mold by further heating with steam, whereby they sinter into a molded body corresponding in its dimensions to the inside of the mold cavity. After this final foaming the molded and shaped article so obtained is cooled within the mold. It must be cooled until the inside of the molded body has dropped to temperatures below the softening point. When the molded article is prematurely removed from the mold, deformation may take place. Because foamed plastics are good insulators, relatively long cooling times are required to cool the molded articles. The time after which a molded article can be ejected at the earliest without thereby incurring subsequent deformation is termed the minimum dwell time (MDT).
After ejection, the molded article is stored for some further time until full cooling is achieved and, when a block for instance is involved, the block is then cut into foamed panels for insulating purposes. During the block storage its side faces collapse more or less, and this phenomenon is observed mostly at those sides which are directly exposed to steaming. The collapse of the blocks and a degraded fusion of the foamed styrene polymer particles especially takes place when a halogenated flame-retardant agent has been added to the styrene polymer. There have been numerous attempts made to optimize the processing parameters of minimum dwell time and fusion grade by providing additives of the most diverse sort. Thus it is recommended to add fatty acids or fatty acid derivatives as disclosed in U.S. Pat. No. 3,389,097 and West German Pat. No. 21 33 253, or rubber, as disclosed in U.S. Pat. No. 3,682,844 and West German Pat. No. 21 01 666. Further, polyethylene waxes are used as disclosed in U.S. Pat. Nos. 3,320,188 and 3,060,138 and European Pat. No. 0 000 120, or bromine compounds as disclosed in French Pat. No. 1 530 701 and West German published application No. 25 42 281 and these additives are recommended to shorten the minimum dwell time but with high degrees of fusion. Again suggested are process-technological steps such as quicker cooling of the beads prior to foaming as disclosed in German Pat. No. 15 04 577. West German published application No. 27 55 005 describes an improved process for preparing expandable styrene polymer particles, the operation being in a reactor completely filled with water. This process is costly in apparatus and the homogeneous mixing of a completely filled polymerizing reactor is not assured in practice. Caking cannot be prevented in such a system, especially where substantially large reactors are involved, and the caking results in a high proportion of unusable beads.
As mentioned above, a pronounced collapse of the sides of the final-foamed blocks is observed, especially for flame-proofed, expandable styrene polymers. To prevent this drawback, a variety of additives such as hydroxylamines as disclosed in U.S. Pat. No. 3,755,209, amines as disclosed in U.S. Pat. No. 4,029,614, or triazine derivatives substituted with amines as disclosed in U.S. Pat. No. 4,192,922 have been proposed. Supposedly, too, the addition of bromine compounds copolymerizing with styrene improves the dimensional stability of the foamed blocks as disclosed in U.S. Pat. No. 4,086,191. However, all the additives suffer from the drawback that they enter into unmanageable side-reactions with the other polymerization accessories such as initiators and thereby contribute to poor product reproducibility. Moreover, the additives used in many cases are physiologically objectionable, thereby restricting the product applicability.